JPS61186417A - Heat treatment of steel material - Google Patents

Abstract

PURPOSE:To remove very easily scale from the surface of stainless steel after heat treatment by sticking a specified alkali or salt to the surface of the steel before heat treatment. CONSTITUTION:When Cr stainless steel is subjected to heat treatment such as annealing, an alkali such as NaOH, KOH, Ca(OH)2 or Mg(OH)2 or a salt such as Na2CO3, K2CO3, NaNO3, KNO3 or NaCO is stuck to the surface of the steel, and the steel is annealed at 500-1,200 deg.C. The oxide of the Cr stainless steel produced by the heat treatment reacts with the alkali or salt, forming a soluble compound, so scale can be easily removed by dissolution in an ordinary descaling soln. contg. an HNO3-HF mixture, HCl or H2SO4 without requiring the conventional modification of scale.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention mainly relates to a method of heat treatment of steel materials in which stainless steel is subjected to heat treatment such as annealing.

[Conventional technology]

Conventionally, when heat treatment such as annealing is applied to steel materials such as stainless steel, scale is formed on the surface of the steel material, so in order to remove this scale, HNO3-HF mixed solution, H(J.

The steel material after heat treatment was immersed in a descaling liquid such as H2SO4.

[Problem that the invention seeks to solve]

However, scale is insoluble in these descaling solutions, and a scale modification step is required to make the scale soluble in the descaling solution prior to the above descaling step. Such a scale modification process includes, for example, a sodium bath in which the steel material after KM treatment is immersed in a NaH salt bath.
The Hydride method and the Hooker method in which the steel material is immersed in a Na0H-NaNOa salt bath have been proposed, but these methods require a large amount of energy to produce molten salt and require large-scale equipment.

[Means for solving problems]

The present invention solves the above-mentioned conventional problems by attaching alkali or salts to the surface of the steel material prior to heat treatment.Thus, in the present invention, the scale generated by heat treatment is inherently soluble in the descaling agent. Therefore, descaling can be easily performed without any need for a scale modification process.

The invention will be explained in detail below.

The alkali used in the present invention is sodium hydroxide,
Common inorganic alkalis such as potassium hydroxide, calcium hydroxide, and magnesium hydroxide, and salts include sodium carbonate, potassium carbonate, sodium nitrate, potassium nitrate, and sodium chloride.

Common metal salts such as potassium chloride, sodium sulfate, and potassium sulfate are exemplified. Two or more kinds of the alkalis or salts mentioned above may be mixed, or a mayu alkali and a salt may be mixed. The above-mentioned alkali and/or salts are usually made into an aqueous solution, and the steel material is immersed in the aqueous solution, or the aqueous solution is applied to the surface of the steel material by spraying, brushing, etc.

The steel material thus surface-treated is subjected to a predetermined heat treatment such as annealing. The heat treatment temperature varies depending on the type of steel material to be treated, but for example, in the case of chromium stainless steel, it is
The range is preferably 00 to 1200°C, more preferably 650 to 11
A range of 00°C is desirable.

[Operation] As mentioned above, when heat treatment is performed with alkali and/or salts attached to the surface of the steel material, oxides of the metals constituting the steel material react with the alkali or salts to form soluble compounds. becomes. For example, in the case of chromium stainless steel, chromium oxide Cr2O3 which is scale
is solubilized by reacting with sodium hydroxide as shown in the following equation.

CrzO3+2NaOH4NazCrzOn+HzO In the above reaction, at a temperature below 500''C, the reaction rate is slow and the amount of soluble product NazCrz04 produced is small, and at a temperature above 1200°C, NazCr204 liquefies and the progress of the reaction is inhibited.

When 5 volumes or more of oxygen is present in the atmosphere of the furnace during the heat treatment, the scale solubilization reaction described above is promoted.

The scale solubilized in the above heat treatment can be easily dissolved and removed immediately after the heat treatment using a common descaling solution such as Group 03-HF mixed solution, HCl, H2SO4, etc.

〔effect〕

As described above, the present invention is capable of attaching alkali and/or salts to the surface of steel materials using extremely simple equipment such as dipping, spraying, or brush coating, and the energy required for the attachment is also extremely small. It is. Furthermore, since the scale is solubilized at the same time as the heat treatment, there is no need to carry out another scale solubilization step after the heat treatment, which shortens the process. In this way, the present invention greatly streamlines the heat treatment process for steel materials, and the method of the present invention also makes it possible to easily solubilize the scale of chromium stainless steel, which was conventionally difficult to solubilize. It can be removed.

Examples for explaining the present invention in more detail will be described below.

〔Example〕

SUS 304 and SUS 430 are used as the steel materials. The above steel material was cut into a cylindrical shape with a diameter of 5.51) I and a length of 60 ff to prepare a sample. Each sample was immersed in an aqueous sodium carbonate solution for 2 minutes at room temperature, and then dried at 100°C for 10 minutes. After the sodium carbonate was deposited on the surface in this manner, the sample was then placed in a heat treatment furnace and heat treated at a predetermined temperature in an argon-oxygen mixed atmosphere for one hour. After heat treatment, cool the sample and then add 3% by weight of HF and 15% by weight of HNO.
The sample was immersed in a mixed solution at 40°C for 5 minutes to dissolve and remove the scale generated on the sample surface during the heat treatment process. Thereafter, the sample surface was brushed, washed with water, dried, and the sample surface was visually observed to evaluate the descaling property.Table 1 shows the comparison results of the descaling property of SUS 304 and 5US430.

Table 1 The data in Table 1 is heat treatment temperature: 750°C, oxygen concentration: 10
This relates to heat treatment carried out under conditions of low capacity. According to Table 1, it is clear that the descaling property is not affected by the type of stainless steel, which shows that the method of the present invention allows different types of steel materials to be heat treated on the same line.

Next, Table 2 shows the descaling properties of 5US430 when the heat treatment temperature was set to 750°C and the oxygen concentration in the heat treatment atmosphere was varied. Shows desired results when .

Next, the amount of Na 2 Cr 2 O 4 produced per sample (1 q
) is shown in Figure 1. If you look at Figure 1, the heat treatment temperature is 50
It can be seen that the amount of solubilized scale produced increases rapidly at temperatures above 0°C, especially above 650°C, and below 1200°C, especially below 1100°C.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the heat treatment temperature ('C) and the amount of solubilized scale produced. Patent applicant: Daido Steel Co., Ltd. 4005006007C0800900100011C
X) 1201) 1300 heat treatment temperature (°C)

Claims (1)

[Claims] A method for heat treatment of steel material, which comprises introducing alkali and/or salts onto the surface of the steel material and then introducing the material into a heat treatment furnace.